Method of controlling a mobile heating device

ABSTRACT

A method of controlling a mobile heating device which is adapted for generating heat by combustion of fuel with combustion air is provided, the method comprising the steps: operating the mobile heating device with a predetermined fuel supply rate and a predetermined combustion air supply rate, monitoring the signal of at least one temperature sensor over time, temporarily changing at least one of the fuel supply rate and the combustion air supply rate, analyzing the response in the signal of the at least one temperature sensor, and based on the response, operating the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate.

The present invention relates to a method of controlling a mobile heating device and to a mobile heating device.

Heating devices for mobile use or mobile applications (in the following: mobile heating devices) are particularly used in vehicles as parking heaters or additional heaters.

In such heating devices, typically fuel and combustion air is combusted in order to generate heat. To achieve this, mobile heating devices typically comprise a combustion chamber in which combustion takes place. With the requirement to develop vehicles showing reduced emissions, a demand for developing mobile heating devices releasing fewer emissions comes along. In general, the amount of emissions correlates with the ratio of combustion air to fuel. The air-fuel-ratio AFR is the ratio between the mass of air and the mass of fuel in the mixture which is combusted. Commonly a combustion process is categorized in relation to a stoichiometric combustion in which exactly the mass of air is present which is need for entire combustion of the fuel. To this end, the ratio λ (called λ value in the following) of the actual air-fuel-ratio to the stoichiometric air-fuel-ratio (λ=AFR_(actual)/AFR_(stoichiometric)) is used to characterize the actual combustion taking place. Thus, λ>1 defines a combustion with excess of combustion air, λ<1 defines a combustion with deficient combustion air, and λ=1 defines stoichiometric combustion. In particular, a combustion process with λ<1 comes along with increased emissions, since the fuel cannot be completely combusted. Further, the temperature which can be attained in the combustions chamber has a maximum at λ=1 and becomes lower to higher or lower values of λ. Thus, it is an aim to operate mobile heating devices in a range of λ>1 in order to achieve an efficient combustion with low emissions. Typically mobile heating devices are designed to operate in the range 1.2≦λ≦1.8. However, in applications of mobile heating devices, problems in the supply of combustion air can occur which result in reduced mass flow of combustion air and thus undesired air-fuel-ratios. Mobile heating devices typically do not comprise a lambda sensor (air-fuel ratio gauge) and typically the mass flow of fuel and the mass flow of combustion air are not directly measured either. As a consequence, undesired air-fuel ratios cannot easily be detected.

It is an object of the present invention to provide an improved method of controlling a mobile heating device and an improved mobile heating device.

The object is attained by a method of controlling a mobile heating device which is adapted for generating heat by combustion of fuel with combustion air, the method comprising the steps: operating the mobile heating device with a predetermined fuel supply rate and a predetermined combustion air supply rate; monitoring the signal of at least one temperature sensor over time; temporarily changing at least one of the fuel supply rate and the combustion air supply rate; analyzing the response in the signal of the at least one temperature sensor; and based on the response, operating the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate.

By temporarily changing at least one of the fuel supply rate and the combustion air supply rate and analyzing the response in the signal of the temperature sensor, information about the actual air-fuel ratio can easily be deduced without requiring a costly lambda sensor and without measuring the mass flow of fuel and the mass flow of combustion air. Thus, if it is found based on the analyzed response that the mobile heating device is operated with an undesired λ value, the mobile heating device can be operated with at least one of an adapted fuel supply rate and an adapted combustion air supply rate such that combustion is shifted to the desired air-fuel ratio. If analysis of the response results in that the mobile heating device is already operated with the desired air-fuel ratio, of course neither the fuel supply rate nor the combustion air supply rate is changed. As a consequence, the method of controlling a mobile heating device enables operating a mobile heating device more reliable and with reduced emissions. Further, plugging of a combustion air supply line can quickly and easily be detected in which case the mobile heating device can be shut down, for example. Preferably, temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises temporarily changing the air-fuel ratio. Preferably, the fuel supply rate is temporarily changed while the combustion air supply rate is maintained unchanged. In particular, the method can be realized such that, based on the response, the mobile heating device is operated with at least one of an adapted fuel supply rate and an adapted combustion air supply rate only if analyzing the response results in that an undesired air-fuel ratio is present.

According to a further development, operating the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate comprises operating the mobile heating device with an adapted air-fuel ratio. In this case, the mobile heating device can conveniently be operated with a desired X, value.

According to a further development, analyzing the response comprises attributing the response to one of an initial λ value which is equal to or higher than 1 and an initial λ value lower than 1. In this case, based on the result it can easily be decided in which way a ratio of the combustion air supply rate to the fuel supply rate should be changed in order to operate the mobile heating device with low emissions.

According to a further development, operating the mobile heating device with an adapted air-fuel rate comprises: if the initial λ value is found to be lower than 1, operating the mobile heating device with an increased air-fuel ratio. In this case, the mobile heating device can conveniently be operated with reduced emissions. As an alternative, if the initial A, value is found to be lower than 1 it is also possible to shut down the mobile heating device in order to prevent operation with increased emissions.

According to a further development, temporarily changing the at least one of the fuel supply rate and the combustion air supply rate comprises abruptly changing the at least one of the fuel supply rate and the combustion air supply rate for a predetermined time. In this context, temporarily changing means that the respective supply rate is changed only for a predetermined period and thereafter is returned to the previous value. Abruptly changing the fuel supply rate or the combustion air supply rate results in a clear response in the signal of the temperature sensor which can easily be detected and analyzed.

According to a further development, temporarily changing the at least one of the fuel supply rate and the combustion air supply rate comprises changing one of the fuel supply rate and the combustion air supply rate in the manner of a square wave signal. In this case, a particularly clear response is attained. A particularly clear response can be attained if the fuel supply rate is changed in this manner.

According to a further development, temporarily changing the at least one of the fuel supply rate and the combustion air supply rate comprises abruptly changing the air-fuel ratio. An abruptly changed air-fuel ratio results in a particularly easily detectable response.

According to a further development, the at least one temperature sensor comprises an exhaust gas temperature sensor for measuring the temperature of exhaust gas discharged from a combustion chamber of the mobile heating device. Since the signal of an exhaust gas sensor correlates with and follows the temperature in the combustion chamber, analyzing the response in the signal of an exhaust gas sensor allows particularly reliable analysis. For example, the exhaust gas sensor may be arranged directly downstream of the combustion chamber, e.g. at an entry of a heat exchanger. However, it is also possible that the exhaust gas sensor is arranged further downstream, e.g. even downstream of the heat exchanger, for example in an exhaust gas outlet of the mobile heating device.

According to a further development, temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises temporarily reducing one of the fuel supply rate and the combustion air supply rate by a factor of 0.67 or lower. Reducing by a factor of 0.67 means that a supply rate is applied which corresponds to the initial supply rate multiplied by a factor of at most 0.67. In this case, a pronounced response which can clearly be attributed to an initial λ value above or below 1 is achieved.

According to a further development, temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises temporarily enhancing one of the fuel supply rate and the combustion air supply rate by a factor of 1.3 or higher. Again, preferably the fuel supply rate is temporarily changed in order to achieve a particularly pronounced response. Temporarily enhancing the respective supply rate by a factor of 1.3 or higher means that a supply rate is applied which corresponds to the initial supply rate multiplied by a factor of at least 1.3.

The object is also attained by a mobile heating device comprising: a combustion chamber for combusting fuel with combustion air; a fuel supply for supplying fuel to the combustion chamber with a fuel supply rate; a combustion air supply for supplying combustion air to the combustion chamber with a combustion air supply rate; at least one temperature sensor for detecting a temperature of the mobile heating device; and a control unit for controlling operation of the mobile heating device, wherein the control unit is adapted such that it: operates the mobile heating device with a predetermined fuel supply rate and a predetermined combustion air supply rate; monitors the signal of the at least one temperature sensor over time; temporarily changes at least one of the fuel supply rate and the combustion air supply rate; analyzes the response in the signal of the at least one temperature sensor; and based on the response, operates the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate. The thus-adapted mobile heating device achieves the advantages which have been described above with regard to the method of controlling a mobile heating device. In particular, by temporarily changing at least one of the fuel supply rate and the combustion air supply rate and analyzing the response in the signal of the temperature sensor, information about the actual air-fuel ratio can easily be deduced without requiring a costly lambda sensor and without measuring the mass flow of fuel and the mass flow of combustion air. Thus, based on the analyzed response, the mobile heating device can be operated with at least one of an adapted fuel supply rate and an adapted combustion air supply rate such that combustion is shifted to the desired air-fuel ratio. As a consequence, the mobile heating device can be operated more reliable and with reduced emissions. Further, plugging of a combustion air supply line can quickly and easily be detected in which case the mobile heating device can be shut down. Preferably, temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises temporarily changing the air-fuel ratio. Preferably, the fuel supply rate is temporarily changed while the combustion air supply rate is maintained unchanged.

In the present context, the term mobile heating device is used to characterize a heating device which is adapted such that it is suited for mobile applications. This means that the heating device is transportable (e.g. incorporated into a vehicle or placed therein for transport purposes) and is not solely adapted for a permanent stationary use as is the case for a domestic heating device, for example. The mobile heating device can in particular be adapted for heating a vehicle or a temporarily stationary application such as big tents or mobile cabins (welfare cabins). Thus, the term mobile heating device is also used for a heating device which is adapted for fixed installation in a mobile device (such as a vehicle or a mobile cabin). In particular, the mobile heating device can be adapted as a parking heater or additional heater for a motor vehicle.

According to a further development, operating the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate comprises operating the mobile heating device with an adapted air-fuel ratio. In this case, the mobile heating device is adapted to operate with a desired λ value.

According to a further development, analyzing the response comprises attributing the response to one of an initial λ value which is equal to or higher than 1 and an initial λ value lower than 1. In this case, the control unit is adapted to distinguish between operation resulting in undesirably high emissions and the intended low emission operation and can initiate appropriate countermeasures in the case of operation with high emissions.

According to a further development, operating the mobile heating device with an adapted air-fuel ratio comprises: if the initial λ value is found to be lower than 1, operating the mobile heating device with an increased air-fuel ratio. In this case, if combustion with high emissions is found, the operation of the mobile heating device is adjusted to lower emission operation.

Instead of operating the heating device with an increased air-fuel ratio it is also possible to adapt the control unit such that it shuts down the heating device in the case of an initial λ value which is below 1.

According to a further development, the at least one temperature sensor comprises an exhaust gas temperature sensor for measuring the temperature of exhaust gas discharged from the combustion chamber. The exhaust gas temperature sensor allows very fast and efficient analysis since the temperature of the exhaust gas correlates well with the temperature in the combustion chamber.

Further features and advantages will become apparent from the following description of an embodiment which will be described with reference to the enclosed drawings.

FIG. 1 is a schematic illustration of a mobile heating device.

FIG. 2 is a schematic illustration of a typical response according to the embodiment in the case of an initial λ value which is higher than 1.

FIG. 3 is a schematic illustration of a typical response according to the embodiment in the case of an initial λ value which is lower than 1.

FIG. 4 is a schematic flow chart of a method of controlling a mobile heating device according to an embodiment.

An embodiment will now be described with reference to FIGS. 1 to 3. FIG. 1 is a schematic illustration of a mobile heating device according to an embodiment. In the embodiment that will be described in the following, the mobile heating device 1 is adapted as a parking heater and/or additional heater for a motor vehicle. The mobile heating device 1 can for example be realized as a liquid heater which is adapted to heat a liquid, e.g. cooling liquid in a cooling liquid circuit of an engine of a vehicle, as a medium to be heated. As another example, the mobile heating device 1 can also be realized as an air heater which is adapted to heat air as a medium to be heated. Mobile heating devices 1 of this kind are well known in the art and comprise a combustion chamber 2 in which combustion air and fuel are mixed and combusted in order to release heat which is used for heating the medium to be heated.

As schematically shown in FIG. 1, the mobile heating device 1 according to the embodiment is realized as a so-called evaporation burner comprising an evaporator body 3 for evaporating liquid fuel which is supplied by a fuel supply 4 (schematically indicated). The liquid fuel may for example correspond to the liquid fuel which is also used for the engine of the motor vehicle. The fuel supply 4 may in particular comprise a fuel pump (not shown) which is adapted for supplying fuel to the evaporator body 3 with a predetermined fuel supply rate as schematically depicted by an arrow. According to the embodiment, the fuel supply 4 is adapted such that fuel can be supplied at different fuel supply rates which correspond to different heating power levels of the mobile heating device 1. The mobile heating device 1 further comprises a combustion air supply 5 (schematically indicated) for supplying combustion air to the combustion chamber 2 with a combustion air supply rate, as schematically depicted by arrows in FIG. 1. The combustion air supply 5 may comprise a fan or blower (not shown) which can be operated at different speed levels in order to provide different air supply rates. Both the fuel supply 4 and the combustion air supply 5 may preferably be adapted to enable continuously adjusting the respective supply rates. However, it is also possible to realize the fuel supply 4 and/or the combustion air supply 5 such that only discrete levels of the respective supply rate can be set.

A heat exchanger 6 through which the hot exhaust gases are guided is arranged downstream of the combustion chamber 2. The hot exhaust gases from the combustion chamber 2 are guided in contact with an inner wall of the heat exchanger 6 as schematically depicted by arrows in FIG. 1. The medium to be heated is guided between the opposite side of the inner wall and an outer wall of the heat exchanger 6, as also schematically depicted in FIG. 1. In the heat exchanger 6, heat is transferred from the hot exhaust gases via the inner wall to the medium to be heated. After having passed the heat exchanger 6 the exhaust gases are discharged through an exhaust gas outlet 7. The mobile heating device 1 is further provided with at least one temperature sensor which is adapted and arranged for measuring a temperature which is characteristic for the combustion process taking place in the combustion chamber 2. In the embodiment illustrated in FIG. 1, the temperature sensor is formed by an exhaust gas temperature sensor 8 for measuring the temperature of the exhaust gas discharged from the combustion chamber 2 of the mobile heating device 1. The exhaust gas temperature sensor 8 is arranged in thermal contact with the exhaust gases discharged from the combustion chamber 2. Although an arrangement of the exhaust gas temperature sensor 8 in close proximity to the exhaust gas outlet 7 is schematically shown in FIG. 1, such an exhaust gas temperature sensor 8 may also be arranged at a different position, e.g. closer to the combustion chamber 2 with regard to the flow path of the exhaust gases. Further, although the temperature sensor is realized as an exhaust gas temperature sensor 8 in the embodiment (which is a preferred realization), it is also possible to use one or more other temperature sensors for detecting a temperature which is characteristic for the combustion process in the combustion chamber 2. Further, it is also possible to analyze the signals of the exhaust gas temperature sensor 8 and of one or more other temperature sensors.

The mobile heating device 1 is provided with a control unit 10 which is adapted for controlling operation of the mobile heating device 1. To this end, the control unit 10 is connected to the at least one temperature sensor such that it receives a signal representing a temperature which is characteristic for the combustion process in the combustion chamber 2. The control unit 10 is further connected to the fuel supply 4 such that it can control the fuel supply rate and is connected to the combustion air supply 5 such that it can control the combustion air supply rate.

In the embodiment, in normal operation the control unit 10 operates the mobile heating device 1 at a certain heating power level which can e.g. be defined by a certain predetermined fuel supply rate. For such heating power level, the control unit 10 operates the combustion air supply 5 with a predetermined combustion air supply rate which is set such that—under normal conditions—a λ value in a predetermined range is expected (e.g. 1.2≦λ≦1.8) which corresponds to operation with low emissions.

A method of controlling the mobile heating device 1 according to the embodiment will now be described with reference to FIGS. 2 to 4. In FIGS. 2 and 3, the fuel supply rate sr and the signal T of the temperature sensor (which in the present embodiment is formed by the exhaust gas temperature sensor 8) are shown in arbitrary units as a function of time.

As can be seen in FIG. 2 and FIG. 3, in a first step S1 up to a time t₁ the fuel supply rate sr (broken line) is maintained on a constant level sr₀ (initial fuel supply rate) which corresponds to a predetermined heating power level of the mobile heating device 1. The combustion air supply rate is not shown in FIGS. 2 and FIG. 3, but according to the embodiment, the combustion air supply rate is maintained on a constant level (initial combustion air supply rate) during the depicted time period.

The signal T of the temperature sensor is monitored by the control unit 10 (step S2). In the present embodiment, the signal T of the temperature sensor is e.g. sampled every 0.1 seconds or every second.

At the time t₁, the fuel supply rate sr is abruptly reduced to a lower level sr_(x), with sr_(x)≦0.67×sr₀, as can be seen in FIGS. 2 and 3. In other words, the fuel supply rate sr is abruptly reduced by at least one third. At a time t₂ which is several seconds after the time t₁, e.g. t₂−t₁=10 s, the fuel supply rate sr is abruptly returned to the initial fuel supply rate sr₀. Thus, in the embodiment shown, in a third step S3 the fuel supply rate sr is temporarily changed in the manner of a square wave signal for a predetermined time period from t₁ to t₂.

As can be seen in FIGS. 2 and 3, a clear response to this temporary change in the fuel supply rate sr occurs in the signal T of the at least one temperature sensor, in particular if the temperature sensor is formed by the exhaust gas temperature sensor 8. As can also clearly be seen in FIGS. 2 and 3, the response clearly differs in the case where the initial λ value was larger than or equal to 1 (λ≧1; FIG. 2) as compared to the case where the initial λ value was lower than 1 (λ<1; FIG. 3). Thus, the response in the signal T of the temperature sensor can be used to assess if the initial air-fuel ratio was such that λ>1 or such that λ<1. According to the embodiment, the control unit 10 is adapted to analyze the response in the signal T of the temperature sensor and to assess if the initial λ value was equal to or higher than 1 or if the initial λ value was lower than 1. This is done in a step S4. Analyzing the response by the control unit can e.g. encompass analyzing of one or more temperature gradients, comparison of the signal to pre-set threshold values and the like. As is evident for a skilled person, many suitable techniques for analyzing the response in the signal T of the temperature sensor are available.

In the embodiment shown, in a step S5, based on the response in the signal T of the temperature sensor, the control unit 10 operates the mobile heating device 1 with an adapted fuel supply rate and/or with an adapted combustion air supply rate. For example, if the analysis results in that the initial λ value was below 1, the control unit 10 can preferably operate the mobile heating device 1 with unchanged fuel supply rate but with an adapted combustion air supply rate which is increased as compared to the initial combustion air supply rate in order to shift the λ value to a higher value. Alternatively, if an initial λ value lower than 1 is found, the control unit could also judge that a defect in the combustion air supply is present and shut down operation of the mobile heating device 1. On the other hand, if an initial λ value which is in the desired range is found, the control unit 10 operates the mobile heating device 1 with unchanged combustion air supply rate and unchanged fuel supply rate.

Now, the reason for the difference in the response in the signal T of the temperature sensor will be described. First, the response for the case of an initial λ value equal to or higher than 1 will be described with reference to FIG. 2.

If the initial λ value is equal to or higher than 1, in step S1 the fuel supplied to the combustion chamber 2 is completely combusted which results in a high temperature in the combustion chamber and thus a high temperature represented by the signal T. At the time t₁, the amount of fuel which is supplied to the combustion chamber 2 is abruptly reduced such that less fuel for combustion is made available to the combustion process. As a result, the temperature in the combustion chamber 2 and thus the signal T of the temperature sensor drops in response to the temporary change in the fuel supply rate. After the initial fuel supply rate is restored at time t₂, the temperature in the combustion chamber 2 and the signal T of the temperature sensor return to their initial higher values, as can be seen in FIG. 2.

If however the initial λ value is lower than 1, as depicted in FIG. 3, in step S1 the fuel supplied to the combustion chamber 2 is not completely combusted. As a consequence, the temperature in the combustion chamber 2 and thus the signal T of the temperature sensor is lower as compared to a case in which complete combustion would take place. Further, in this case excess fuel is present in the evaporator body 3 due to the deficiency in combustion air. At the time t₁, the fuel supply rate is abruptly reduced such that less additional fuel is supplied to the combustion chamber 2. Since the amount of combustion air supplied to the combustion chamber 2 is maintained unchanged, the actual air-fuel ratio is shifted to a higher λ value. Further, the excess fuel which is present in the evaporator body 3 is now evaporated more efficiently. This means that more complete and thus more efficient combustion takes place which results in an increased temperature in the combustion chamber 2 as represented by the signal T of the temperature sensor, as can be seen in FIG. 3.

Thus, according to the embodiment described above, a method of controlling a mobile heating device 1 and a mobile heating device 1 which is correspondingly adapted are disclosed which enable assessing the air-fuel ratio present in the combustion chamber 2 in an indirect manner without requiring one or more additional sensors. In turn, this enables controlling the mobile heating device 1 such that operation with an increased level of emissions can be reliably prevented.

Modifications of the Embodiment

Although an embodiment has been described in which the fuel supply rate is temporarily reduced by a certain factor while the combustion air supply rate is maintained constant, it is also possible to temporarily increase the fuel supply rate while the combustion air supply rate is maintained constant. This would also result in a response in the signal T of the at least one temperature sensor which can be clearly attributed to either one of an initial λ value of equal to or larger than 1 or an initial λ value smaller than 1. Preferably, in this case the fuel supply rate is abruptly increased by a factor of 1.3 or higher (i.e. by at least thirty percent) in order to achieve a clear response. Again, a temporary change having the shape of a square wave signal is particularly preferred.

Similarly, instead of temporarily changing the fuel supply rate while maintaining the combustion air supply rate unchanged, it is also possible to temporarily change the combustion air supply rate while maintaining the fuel supply rate unchanged. In the case of an increase in the combustion air supply rate, a similar response as in the case of a reduction in the fuel supply rate can be observed. On the other hand, the response to an abrupt decrease in the combustion air supply rate is somehow similar to the response to the abrupt increase in the fuel supply rate. In these cases again, a reduction by a factor of 0.67 or lower or an increase by a factor of 1.3 or higher is preferred.

Further, combined schemes for the temporary change in which both the fuel supply rate and the combustion air supply rate are temporarily changed are also possible.

Although an embodiment has been described in which a rather general distinction is made between, on the one hand, an initial λ value which is equal to or higher than 1 and, on the other hand, an initial λ value which is lower than 1, the response in the signal T of the temperature sensor can be analyzed even more detailed in order to evaluate the value of the air-fuel ratio more in detail. Further, it is also possible to analyze the signal of more than one temperature sensor.

According to a further development, the step of temporarily changing at least one of the fuel supply rate and the combustion air supply rate and analyzing the response can also be repeated several times. In this case the response can be analyzed by statistical methods in order to achieve an even more detailed and/or more reliable result. According to an even further development, the step of temporarily changing at least one of the fuel supply rate and the combustion air supply rate is be repeated several times but with different amounts of change, i.e. with different intensities. By analyzing the responses to such changes, the initial λ value can be assessed with high accuracy more in detail.

The disclosed method can e.g. be conducted each time the mobile heating device 1 is started at a predetermined point in time after start or in regular or irregular intervals during the life-time of the mobile heating device 1.

The described method of controlling a mobile heating device and the correspondingly adapted mobile heating device enable assessing the quality of combustion in the combustion chamber without requiring additional sensors. Defects of the combustion air supply and plugging of the combustion air supply line can reliably be detected. Further, analysis can be integrated easily in the control unit and, if desired, more detailed analysis is also possible. 

1. A method of controlling a mobile heating device which is adapted for generating heat by combustion of fuel with combustion air, the method, comprising the steps of: operating the mobile heating device with a predetermined fuel supply rate and a predetermined combustion air supply rate; monitoring a signal from at least one temperature sensor over time; temporarily changing at least one of the fuel supply rate and the combustion air supply rate; analyzing a response in the signal from the at least one temperature sensor; and based on the response, operating the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate.
 2. The method according to claim 1, wherein the step of operating the mobile heating device with the at least one of an adapted fuel supply rate and an adapted combustion air supply rate comprises the step of operating the mobile heating device with an adapted air-fuel ratio.
 3. The method according to claim 2, wherein the step of analyzing the response comprises attributing the response to one of an initial λ value which is equal to or higher than 1 and an initial λ value lower than
 1. 4. The method according to claim 3, wherein the step of operating the mobile heating device with an adapted air-fuel rate comprises the step of operating the mobile heating device with an increased air-fuel ratio if the initial λ value is found to be lower than
 1. 5. The method according to claim 1, wherein the step of temporarily changing the at least one of the fuel supply rate and the combustion air supply rate further comprises the step of abruptly changing the at least one of the fuel supply rate and the combustion air supply rate for a predetermined time.
 6. The method according to claim 1, wherein the step of temporarily changing the at least one of the fuel supply rate and the combustion air supply rate further comprises the step of changing one of the fuel supply rate and the combustion air supply rate in the manner of a square wave signal.
 7. The method according to claim 1, wherein the step of temporarily changing the at least one of the fuel supply rate and the combustion air supply rate further comprises the step of abruptly changing the air-fuel ratio.
 8. The method according to claim 1, wherein the step of monitoring at least one temperature sensor comprises the step of monitoring an exhaust gas temperature sensor for the step of measuring the temperature of exhaust gas discharged from a combustion chamber of the mobile heating device.
 9. The method according to claim 1, wherein the step of temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises the step of temporarily reducing one of the fuel supply rate and the combustion air supply rate by a factor of 0.67 or lower.
 10. The method according to claim 1, wherein the step of temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises the step of temporarily enhancing one of the fuel supply rate and the combustion air supply rate by a factor of 1.3 or higher.
 11. A mobile heating device comprising: a combustion chamber for combusting fuel with combustion air; a fuel supply for supplying fuel to the combustion chamber with a fuel supply rate; a combustion air supply for supplying combustion air to the combustion chamber with a combustion air supply rate; at least one temperature sensor for detecting a temperature of the mobile heating device, and a control unit for controlling operation of the mobile heating device, the control unit being adapted such that it operates the mobile heating device with a predetermined fuel supply rate and a predetermined combustion air supply rate, monitors the signal of the at least one temperature sensor over time, temporarily changes at least one of the fuel supply rate and the combustion air supply rate, analyzes the response in the signal of the at least one temperature sensor; and based on the response, operates the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate.
 12. The mobile heating device according to claim 11, wherein the mobile heating device operates with at least one of an adapted fuel supply rate and an adapted combustion air supply rate comprises the mobile heating device operating with an adapted air-fuel ratio.
 13. The mobile heating device according to claim 12, wherein the device analyzes the response by attributing the response to one of an initial 2 value that is equal to or higher than 1 and an initial 2 value lower than
 1. 14. The mobile heating device according to claim 13, wherein the mobile heating device operating with an adapted air-fuel ratio comprises operating the mobile heating device with an increased air-fuel ratio if the initial λ value is found to be lower than
 1. 15. The mobile heating device according to claim 11, wherein the at least one temperature sensor comprises an exhaust gas temperature sensor for measuring the temperature of exhaust gas discharged from the combustion chamber. 